The present invention relates to a flexible composite release sheet providing a replicative surface with a desired surface effect. More particularly, the present invention relates to a composite release sheet providing a replicative surface with a desired surface effect for use in multicomponent reactive urethane systems. The present invention further relates to a method of manufacturing such a release sheet. The term xe2x80x9csurface effect,xe2x80x9d as used herein, is intended to encompass three dimensional relief patterns, textures, or embossures, and essentially flat finishes, such as highly glossy mirror finish.
A number of processes exist in which a plastic film or sheet is formed on or against a release sheet and then separated from the release sheet after cooling or curing to set the plastic material. Curing, where necessary, may be accomplished by heat, by peroxide catalyst, by ultraviolet (UV) radiation or by electron beam radiation. The release sheet provides a surface from which the set plastic material can be readily separated and imparts to the surface of the plastic material the quality of finish of the release surface. For example, a desired textured surface can be provided on the surface of the plastic material by forming the plastic material on or against a release sheet having a textured surface that is the mirror image of the desired textured surface.
One example of such forming processes is xe2x80x9ccastingxe2x80x9d, wherein a resinous material, such as polyvinyl chloride or polyurethane resin, in a flowable state, is deposited or xe2x80x9ccastxe2x80x9d onto the release sheet surface, heated, cured and cooled to consolidate the resinous material into a continuous self-supporting film, and stripped from the release sheet. The release sheet is normally provided with a desired surface effect, such as high gloss, texturing or an embossed configuration, and the surface effect is replicated on the cast film.
A more specialized casting method involves using multicomponent reactive urethane systems as the resinous material. In general, polyurethane casting materials are formed by reacting a diisocyanate and a polyol to form a urethane. Film properties of the polyurethane can be tailored for specific end-uses by varying the proportions and the chemistry of the diisocyanate and polyol components. Most polyurethanes are used in the xe2x80x9cneatxe2x80x9d or pre-reacted form. Multicomponent reactive urethanes, however, are not pre-reacted before casting. The diisocyanate and polyol components, as well as other additives such as cross-linkers and accelerators, are applied to the substrate as individual entities. The reaction occurs in situ.
Release sheets for use in the above-described casting processes are typically made by coating, treating, or impregnating a paper sheet or other substrate with a release coating comprised of such materials as polymethylpentene, polypropylene, polyfluorocarbons, silicone oil, thermoset silicone resins, and other conventional release agents. Surface effects on the release sheet are conventionally provided by any one of a number of techniques. The release coating can be dried to a smooth surface gloss, or surface effects such as texturing or embossing can be provided in the coating by mechanical means, applied either to the surface of the paper before coating or to the paper after the coating is applied.
U.S. Pat. No. 4,289,821 (Gray et al.) and U.S. Pat. No. 4,322,450 (Gray et al.), the disclosures of which are incorporated herein by reference, disclose techniques for producing surface effects in a release coating on a release sheet for use in casting processes. One method disclosed comprises applying a coating of an electron beam radiation curable material to one surface of a web substrate, pressing the coated side of the substrate against a replicative surface having the desired surface effect to cause the coating to conform to the replicative surface, irradiating the coating with electron beam radiation to cure the coating, and stripping the substrate from the replicative surface with the cured coating adhered to the substrate. The replicative surface is preferably a metal roll with either a pattern engraved in its surface or a highly polished smooth surface. An important advantage of this technique is that the pattern or finish of the replicative surface is reproduced in the cured coating with essentially 100% fidelity. This technique enables replication of very fine patterns, such as wood grain and leather grain, on the surface of a plastic cast onto the release sheet.
U.S. Pat. No. 4,311,766 (Mattor) and U.S. Pat No. 4,327,121 (Gray), the disclosures of which are incorporated herein by reference, disclose electron beam curable coating compositions comprising acrylic functional materials and silicone release agents. Such coatings may be used, e.g., in the processes described in U.S. Pat. Nos. 4,289,821 and 4,322,450, to reproduce a replicative surface in a release sheet with substantially 100% fidelity.
Release sheets using acrylic functional release coatings have been used successfully with casting systems that employ, for instance, polyvinyl chloride or pre-reacted polyurethane. These release sheets, however, typically do not provide adequate release properties when used with multicomponent reactive urethane casting systems.
Some release sheets work well with multicomponent reactive urethane casting systems, such as release sheets manufactured from extruded polypropylene or poly-4-methyl pentene. These release sheets, however, tend to lack the fidelity of replication achieved by the release sheets described in the above mentioned patents. A need remains for release sheets providing a replicative surface with a desired surface effect at substantially 100% fidelity for use in multicomponent reactive urethane casting systems.
The present invention provides flexible composite release sheets providing a replicative surface with a desired surface effect. The invention further provides flexible composite release sheets for use in multicomponent reactive urethane casting systems. The invention further provides flexible composite release sheets having a first acrylic functional coating layer containing the desired surface effect and a second aqueous silicone release coating layer overlying the acrylic functional coating layer. The invention also provides methods for manufacturing such flexible composite release sheets.
In one aspect, the invention provides a flexible composite release sheet comprising:
a) a substrate;
b) an acrylic functional coating layer provided on at least one surface of the substrate, containing said surface effect; and
c) a silicone release coating layer provided on the acrylic functional coating layer, such that the continuity of said release layer is maintained, acceptable release of the cast film from said silicone release coating is achieved and alteration of said underlying desired surface effect is minimized.
Preferred embodiments include one or more of the following features. The substrate is paper. The acrylic functional coating is preferably an electron beam polymerized acrylic functional coating. The acrylic functional coating preferably comprises an acrylated oligomer, a monomer selected from a group consisting of monofunctional acrylate, multifunctional acrylate and mixtures thereof, and 2% or less by total weight of the solid coating of a siloxane release agent. The siloxane release agent is preferably an aminofunctional siloxane release agent. The aminofunctional siloxane release agent is preferably less than 1%, more preferably less than 0.25% and most preferably completely removed from the acrylic functional coating.
The silicone release coating layer preferably comprises 90 parts or less polyvinyl alcohol, 100 parts or less of a complex reactive organofunctional siloxane release modifier, 90 parts or less of a reactive organofunctional siloxane emulsion coating, 10 to 20 parts of a catalyst selected from a group consisting of platinum complex and tin complex, and 2 to 12 parts of a nonionic surface wetting agent containing polyoxyethylene groups. The nonionic surface wetting agent is preferably a silicone glycol copolymer wetting agent. The catalyst is preferably a platinum complex catalyst. The term xe2x80x9cpartsxe2x80x9d as used herein means parts on a dry solids basis.
In another aspect, the invention provides a method of manufacturing a flexible composite release sheet by:
a) applying an acrylic functional coating layer to a substrate;
b) pressing the coated side of the substrate against a replicative surface to cause the coating to conform with the replicative surface;
c) curing the acrylic functional coating layer;
d) stripping the cured coated substrate from the replicative surface;
e) applying a silicone release coating layer on the acrylic functional coating layer; and
f) curing the silicone release coating layer.
Preferred methods include one or more of the following features. The pressing step and curing step are performed simultaneously. The first curing step is preferably achieved by radiation curing, most preferably by electron beam radiation curing. The applying step is preferably performed by airbrush coating.
Other features and advantages of the invention will be apparent from the following detailed description, the drawings, and the claims.